The present invention relates to a drive transmission structure for a tractor comprising a belt type stepless change speed mechanism between an engine and a transmission.
The belt type stepless change speed mechanism employed in this type of drive transmission structure for an agricultural tractor includes two split pulleys one of which engages a belt extending from the engine and the other engages a belt extending to the transmission. The two split pulleys are shiftable toward the engine or toward the transmission, to relax tension of one of the belts and increase the pitch circle diameter of one of the split pulleys and to increase tension of the other belt and reduce the pitch circle diameter of the other split pulley. This provides belt type stepless speed changing.
Further, by shifting the split pulleys to an end of their shifting range, one of the belts is relaxed to be out of contact with a V-groove whereby power transmission is broken. In other words, this construction acts as a clutch as well.
However, since the split pulleys function as a main clutch at the end of their shifting range, the split pulleys must move through change speed positions to arrive at the end of the shifting range to provide a disengaged state of the main clutch. This feature is unsatisfactory from the point of view of operating speed; it is not well adapted to occasions needing to stop the tractor suddenly.